Catalytic cracking is a refining process which is applied commercially on a large scale. In this process heavy hydrocarbon fractions are converted into lighter and less viscous products. The conversion or cracking takes mainly place in the vapor phase in the presence of a catalyst. The feedstock is so converted into e.g. gasoline. Part of the feedstock is converted into gas consisting of four or less carbon atoms per molecule. The gas partly consists of olefins and partly of saturated hydrocarbons.
During the cracking reactions some heavy material, known as coke, is deposited onto the catalyst. This reduces its catalytic activity and regeneration is desired. Regeneration is accomplished by burning off the coke and then the catalyst activity is restored. Therefore, three steps can be distinguished in the process, viz. a cracking step in which the hydrocarbons are converted into lighter products, a stripping step to remove hydrocarbons adsorbed on the catalyst and a regeneration step to burn off coke from the catalyst. The regenerated catalyst is then reused in the cracking step.
As catalysts for catalytic cracking zeolites in an amorphous matrix have been found to be very effective. In particular zeolites having relatively large pores, i.e. having pore diameters larger that 0.7 nm, are very suitable. From No. EP-B-0,131,986 catalyst compositions are known comprising a zeolite I with large pores and a zeolite II, being a ZSM-5-type zeolite, with a pore diameter of smaller than 0.7 nm. Similar catalyst. compositions are also known from U.S. Pat. No. 3,758,403.
As stated hereinbefore, part of the feedstock that has been cracked has been converted into a gaseous fraction Part of this gaseous fraction is saturated and has a relatively low intrinsic value. It would therefore be advantageous if one could amend the process and/or catalyst such that hardly any saturated gaseous compound be formed. Surprisingly, it has now been found that the yield of saturated gaseous components is reduced if the catalyst composition further contains a dehydrogenating metal or metal component.